1. Field of the Invention
The present invention relates to alkoxysilane-functional prepolymers which have been modified to contain allophanate and/or biuret groups having at least one silane-functional or non-functional alkyl or aryl radical, to a process for preparing these prepolymers and to adhesives, sealants, primers or coatings prepared from these prepolymers.
2. Description of Related Art
Alkoxysilane-functional polyurethanes which crosslink via silane polycondensation are well established. A review article on this topic is found for example in “Adhesives Age” 4/1995, page 30 ff. (authors: Ta-Min Feng, B. A. Waldmann). Alkoxysilane-terminated, moisture-curing, one-component polyurethanes are increasingly being used as elastomeric coating, sealing and adhesive compositions in construction and in the automotive industry.
These alkoxysilane-functional polyurethanes can be prepared in accordance with U.S. Pat. No. 3,627,722 or U.S. Pat. No. 3,632,557 by reacting, for example, polyether polyols with an excess of polyisocyanate to form an NCO-containing prepolymer which is then further reacted with an amino-functional alkoxysilane. The resulting alkoxysilane-functional prepolymer contains a high concentration of urea groups and urethane groups, which results in products having high viscosities.
One effective way of reducing at least the portion of the hydrogen bond density resulting from the urea groups is to use secondary aminosilanes to generate substituted ureas. A variety of methods have been proposed for this purpose: U.S. Pat. No. 3,627,722 and U.S. Pat. No. 3,632,557 use alkyl-substituted aminosilanes; U.S. Pat. No. 4,067,844 uses an addition reaction of acrylates with the primary aminosilane; EP-A 596 360 uses an addition reaction of maleic esters with the primary aminosilane; and EP-A 676 403 introduces aryl-substituted aminosilanes. All of these methods, however, are able to replace only one hydrogen atom on the terminal urea group; all other urea protons and urethane protons continue to contribute, via hydrogen bonds, to a high viscosity.
Another appropriate way of reducing the density of the hydrogen bonds and, thus, the viscosity is disclosed in EP-A 372 561, in which very long chain polyether polyols are used with a low level of molecular weight increase through the reaction with the polyisocyanate. This requires polyethers which by virtue of specific preparation processes have a high functionality in conjunction with a low level of unsaturation and polydispersity. Further aspects of this technology are described in WO 99/48942 and WO 00/26271. However, this principle has a significant effect only in the case of very long chain prepolymers, designed for low-modulus binders, and even then it is only possible to eliminate some of the hydrogen bond density.
U.S. Pat. No. 4,345,053 teaches another possibility for reducing the urethane density and, thus, the hydrogen bond density by terminating an OH-functional prepolymer with an isocyanate-functional alkoxysilane, which theoretically saves one urea group per termination. However, the OH-functional prepolymer still contains urethane groups resulting from the reaction of a polyether polyol with diisocyanate. These groups, as also disclosed in EP-A 372 561, can be reduced by using specially prepared long-chain polyethers with a low level of unsaturation and polydispersity. Disadvantages of this approach are that 1) the isocyanatosilanes needed for this purpose are compounds of low availability, which are therefore expensive, 2) the compounds have a tendency to undergo side reactions with the OH groups of the polyethers, and 3) here again only some of the viscosity-increasing urethane groups can be eliminated.
The fact that polyisocyanates with allophanate groups have a relatively low viscosity as compared with their non-allophanate counterparts is known in the literature, for example in EP-B 0 682 012.
In the art they are prepared by reacting a monohydric or polyhydric alcohol with large amounts of excess aromatic, aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, U.S. Pat. No. 3,769,318, EP-A 0 000 194 or EP-A 0 712 840). There exclusively di- or polyisocyanates are used, in order to obtain in turn an isocyanate-functional binder. To prevent premature crosslinking it is necessary to use an excess of polyisocyanate, which, after urethanization and allophanatization have taken place, must be removed by means of vacuum distillation. With this approach, a further isocyanate functional group is linked via the allophanate nitrogen.
It is also possible to prepare allophanates indirectly from isocyanate derivatives other than urethanes and isocyanates. EP-A 0 825 211 describes a process for preparing allophanate groups from oxadiazinetriones; another possibility is the opening of uretdiones (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419 and also US-A 2003 0153713) to allophanate groups. Both of these pathways, however, require refined starting materials and lead only to an allophanate product which is rich in by-products. Additionally, the precursors are constructed using exclusively at least difunctional polyisocyanates.
The use of monoisocyanates has already been disclosed in connection with allophanate chemistry. Applications U.S. Pat. No. 5,663,272 and U.S. Pat. No. 5,567,793 use phenyl isocyanate in order to acquire, by reaction with a polyfunctional alcohol, an NCO- and OH-free urethane, which is subsequently modified by allophanatization with specific MDI grades to give a liquid MDI polyisocyanate. In this approach the product contains monomeric diisocyanate prior to further processing.
It is an object of the present invention to provide modified, alkoxysilane-functional polyurethane prepolymers having a significantly reduced viscosity compared with the prior art.
It has now been found that these prepolymers can be prepared very easily by reacting the urethane groups and/or urea groups of a polyurethane prepolymer, which may contain alkoxysilane groups, with monoisocyanates, the reaction involving partial or complete allophanatization and/or biuretization of these groups. The monoisocyanate may itself contain alkoxysilane groups, so that silane-modified prepolymers can also be obtained from alkoxysilane-free polyurethane prepolymers.